Sharing of the optical band in local systems

Abstract
Ways of supporting large numbers (on the order of thousands) of simultaneous high-speed (~10 Mb/s) users on the lowest hierarchical unit of a coherent optical system are investigated. Assuming each channel uses a different optical carrier, a key impairment is laser phase noise, which causes modulated optical carriers, nominally widely separated in frequency, to not be truly spectrally disjoint. To deal with this problem in consideration of the superabundance of optical bandwidth, attention is focused on the region of low spectral utilization. For this region, formulas are derived that describe the performance of three arrangements of optical channels: equally spaced carriers, randomly placed carriers, and randomly placed carriers with spread spectrum. The formulas quantify the great superiority of equally spaced carriers over random placement, despite the highly significant benefit afforded by spread spectrum. Numerical examples are worked out to show this result